My bike was stolen Wed. night so I'm out looking for a new one. Apparently mountain bikes are increasingly sold with 29-inch wheels instead of the old standard, 26-inch ones. At two different bike shops, the salesmen have told me about how bigger wheels let you "go faster" or "cover more ground with the same effort." But I don't understand the physics of that, and think they're just making things up. When you have an 18- or 24-speed bike, you're always covering as much ground as you can with the torque you choose to exert (unless you're in the highest gear). Right? How would wheel size alter the equations?

I can't answer the underlying physics of it, but I do know the differences between a 29er and a 26er mountain bike.
The salesman are basically making things up, but it's not their fault. The bike industry needs to find a way to sell more bikes, but innovating the bicycle is exceptionally difficult. So you get these incremental, borderline-bogus, changes like 29ers which the whole industry throws their weight behind. Particularly in the US - some major manufacturers have dropped 26ers from their hardtail range and 29er HTs are now standard.

Most important and unequivocal difference first - 29ers look better if you're a very tall rider who needs a large frame. Riding an xtra-large frame on 26 inch wheels makes you look like a bear riding a child's bike at the circus. Conversely - if you ride a small frame (like a lot of women for example), then 29ers look like clown bikes. Aesthetics are important, let's face it.

For performance differences - the larger wheel size copes a bit better with low-level technical stuff, fast rocky trails. It seems to enable a bit more momentum to be carried through the trail chatter. It's really marginal though. Loads of head to head comparisons have been done in the MTB press and you're talking a difference of a few seconds over several km, with a pro-rider in the seat. 29ers are now common in pro XC racing, where you get these sort of trails. So there is a tiny performance gain, but this is really driven by the industry needing to sell something that looks new.

The other side of the performance issue is that 29er HTs feel a little more cumbersome on tight, technical trails (true, IME), So the dogma is that a 26er HT is a better bet on more difficult trails. 29er full suspension bikes are not really in the discussion - or they shouldn't be. It's v difficult to accommodate the larger frame size without compromising the suspension design, so existing 29er FS bikes are not that good.

There's a another standard that is vying for market supremacy - 650B, which I think corresponds to approx 27.5 inch wheel size. Some major bike lines have been launched with this recently (e.g. Santa Cruz Heckler, orange 5). There you really are in the realms of complete cynical bullshit.

A well designed suspension set up does mitigate the issues that smaller wheeled machines run into when facing rough terrain, but its a fact that larger wheels handle rough ground better.

As an extreme example, think of the Ordinary - with its very large front wheel - it was (and is) surprisingly able and comfortable on badly made surfaces. The other end of it is the Moulton/Brompton small wheeler - even with suspension they are just plainly not suited to ruts, bumps, rocky trails.

The 29 inch wheel will handle rough terrain better than the standard wheel but will be poorer at direction changes - smaller wheels tend to steer faster.

The difference is pretty small in this case, it can causes compromises in suspension design, but when you have some riders who are well over 6' this is going to be more suited to them.

If you are going new, then you look at the price and spec, if you are going to the used market then you'll get better value on standard machines because they are suddenly less trendy- in this case you will end up with a better machine for your money, and likely better than a cheap 29" machine.

It does not make a huge difference, but the smaller wheels take less energy to get up to speed while with the larger wheels take less energy to counteract the friction of the road on the wheel, thus making you expend less energy on keeping the bike at speed. Thus if you use the bike in a city and you need to stop and go a lot, smaller wheels are slightly more efficient and if you bike longer distances without stopping then larger wheels are.

You can see that it takes more energy to come up to speed with

rotational energy
E = 0.5 * I * w^2
and inertia
I = 0.5 * m * r^2

E = energy needed
w = rotational speed
r = wheel radius

As you can see, the value of E increases when the value of r is increased.

But larger wheels have more mass (heavier) and could have more friction caused by the wind and possibly thicker wheels on the road could also cause more friction. Small wheels require a faster rotation of the chain and thus slightly more energy could be lost to friction. It is very difficult to say and I wouldn't base my purchase off that. You should just do a test drive on several bikes and see which one makes your riding experience more enjoyable.

I've read a lot about this topic as I've been riding recumbent bikes for a long time. (Many recumbent bikes have small wheels, some as small as 16", but some have 700c wheels. This is a topic of endless debate on recumbent bike forums.)

On a perfectly flat and smooth road, the wheel size makes negligible difference. Rolling resistance is mainly a function of tire construction and tire pressure. (However, if the tire construction and pressure are exactly the same, a larger wheel has less rolling resistance.)

On anything other than a perfectly flat pavement, wheel size has a bigger effect: larger wheels have less resistance. A large wheel effectively smoothes out the irregularities in the road. If there's a dip in the road, a smaller wheel sinks deeper into it, which means it takes more energy to climb out of it. If there is a bump, a smaller wheel climbs it at a steeper angle.

Also, you know the early bicycles with huge front wheels? The pneumatic (air-filled) tire hadn't been invented at that time, so a very large wheel was the only way to get a reasonably comfortable ride on solid rubber tires. The conventional ("ordinary") bicycle became popular after John Dunlop invented the pneumatic tire. (Yes, the large wheel also helped achieve a high speed without need of gears and chains, but gears and chains had been invented much earlier. Geared bicycles were invented, but did not become popular until it was combined with pneumatic tires.)

When you have an 18- or 24-speed bike, you're always covering as much ground as you can with the torque you choose to exert (unless you're in the highest gear). Right?

You're exactly right about this. If it was about covering more ground per pedal stroke, they could just have fitted larger chainrings, which would raise the gear ratio. And 16-inch folding bikes ARE fitted with larger chainrings (and sometimes smaller cassettes) for this reason.

Personally, I think it's a lot more likely that the manufacturers just want to standardize on 700c rims to save money.

I don't think it's that. MTB 700c (sorry, 29") rims are designed specifically for MTB use. Usually a bit wider and more beefy than road rims, and often there is no braking surface (since most MTBs nowadays use disc brakes).

But I suspect manufacturers have an incentive to switch to a different standard and proclaiming it to be better, so MTB owners would upgrade to a new model.

The Ordinary very rarely had pneumatic tyres this is the one that many folk would call a 'Penny farthing', I think you probably mean the Safety, which had the frame shape that is recognisable today with two wheels of the same size, and it's these that became more popular with the advent of air filled tyres, plus the essential chain and geared drivetrain.

It looks to me like the wheel size would be more or less relevant, depending on the circumstances in which the machine is being used. Under some riding conditions a larger wheel is better, and under others a smaller wheel would be better.

Bicycle manufacturers have a complex market that they are selling bicycles to, which over time may shift, creating a change in the potential market share in people riding in optimally big-wheel conditions, and this would result in an industry-wide shift to big wheels to increase riding efficiency to that larger market share more likely to use bicycles in those riding conditions. And/or vice versa.

So when the marketing department researches the potential buyers and finds that there is a shift toward users who would benefit by a different size wheel, they will change the size of the wheel to reach that market.

With the same tire, rim make and model, and spoke count, a 26in wheel will be stronger and lighter than the equivalent 29er/700c wheel.

Shorter spokes makes a stronger wheel and the smaller circumference and radius means less weight and less rotational mass so it is faster accelerating.

A 29er/700c wheel has all the weight further away from the hub, so it has both greater rotational mass and weight. It is harder to accelerate but easier to maintain speed. It also has a higher top speed, but how often are you out spinning the big chainring and small cog combo?

26inch wheels seem to be going away and it is too bad. I don't see any benefits to 29er/700c wheels as an urban commuter cyclists. I stocked up on 26inch disc wheelsets and put them on my 700c cyclocross commuter bikes.

Wheel strength and accelerating when commuting is a much greater priority than top speed.

26inch wheels seem to be going away and it is too bad. I don't see any benefits to 29er/700c wheels as an urban commuter cyclists. I stocked up on 26inch disc wheelsets and put them on my 700c cyclocross commuter bikes.

Wheel strength and accelerating when commuting is a much greater priority than top speed.

I would always have said that 26'' MTB wheels are not going anywhere - but I read today that Giant are moving almost their entire range (downhill bike the exception) to 650B. (press release here)

I guess when the biggest mountain bike manufacturer in the world says it's not making any 26'' bikes, that has to have an impact. I don't think 26'' wheelsets will disappear or anything, but it will be interesting to see what the choice will be like in a few years.

Of course, we could all rise up as one and demand an end to this madness - have the bike companies bend their resources to something that might actually improve mountain bike design and fabrication.

Another wrinkle to the 650B standard is that you should be able to get a 26'' wheel in there without changing the geometry / handling too much, so people might continue with their old wheelsets for longer.

Another wrinkle to the 650B standard is that you should be able to get a 26'' wheel in there without changing the geometry / handling too much, so people might continue with their old wheelsets for longer.

If your 26" wheelset is set up for disc brakes.

I had to replace a frameset earlier this year, and frames with v-brake tabs were pretty rare. I doubt 650 bikes will have them.

I just bought a 29er hardtail this week. I'm on the injured list at the moment, so I've only adjusted it and dialed the fork in a bit. When I'm able, I plan to ride it and my 26" HT on the same loop to see if there's much difference. I'm keeping the 26er as a backup or loaner for when a friend wants to try out mountain biking.

I just bought a 29er hardtail this week. I'm on the injured list at the moment, so I've only adjusted it and dialed the fork in a bit. When I'm able, I plan to ride it and my 26" HT on the same loop to see if there's much difference. I'm keeping the 26er as a backup or loaner for when a friend wants to try out mountain biking.

I did the same thing and truthfully I like them both about the same. I do like the fact that the 29er has disc brakes, but since I've converted them both to tubeless (first the 29er, then the 26") I really don't favor one over the other. I like them both. And since I was down to only four bikes before the 29er (from an all time high of seven) it's not like the extra bike is a luxury, it's more of a need.

The penny-farthing (the thing with a five-foot front wheel) was a design due to the fact the pedals were locked to the front wheel like a tricycle. To get anything beyond tricycle speed, the front wheel had to be quite big.

Think of how a shopping cart with tiny wheels will jam and come to a halt with the least little obstacle under the wheel - a small pebble, etc. To go over an obstacle, the key is the angle of the obstacle. To climb a curb or similar obstacle, for example, the axle must pivot up and over around the point where the wheel touches the top corner of the curb.

If the curb is as high as the axle of the wheel, the forward momentum must turn into vertical momentum to be gin the pivot - which usually aint gonna happen.
If the curb is half the radius of the wheel, the vehicle must translate forward momentum into a 45-degree lift.
and so on...
The smaller the angle from vertical that the wheel hits the obstacle, the less forward momentum is turned into lifting momentum.
I just have trouble imagining that in real world cycling, 26 vs 29 makes that much of a difference over obstacles.

(This is why pulling is easier than pushing. In pulling a two-wheel cart, the pivot is at the handle, a point well in front of the obstacle. In pushing, like in bicycles, the pivot point is behind the obstacle.)

My bike was stolen a few months ago and I replaced it with a 29er. Anecdotally, it has reduced the time taken for my commute by about 10%, but then the 26" bike was a full suspension model and the 29" is a hardtail, so that is probably partly responsible.

I do also find it rolls over obstacles much better off road. It gives a much smoother ride than a 26" hardtail, more different than you would think for the relatively small change in wheel size.

The trade off is slightly more weight and slower acceleration, but once you are up to speed you have more momentum.

I love my 29'er but it certainly is a bit more to manage when negotiating technical trails. I'm much more into cross-country riding where you get some speed and the bike's ability to roll over stuff I might have had to maneuver around is well worth it. If I enjoyed the more technical stuff I'd definitely stick with a 26'er.

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